I'm confused since only two coordinates are given but the problem has 3 variables

The "two coordinates" are values of s and t. When s= -2, t= 2, x= (-2)(2)= -4, , and z= 2^2= 4. Actually, it is a little ambiguous as to which of "(-2, 2)" is s and which t but it turns out that, in this problem, it doesn't matter!